feat: enhance usage and cost tracking accuracy

Improved extraction of reasoning and cached tokens from OpenAI and DeepSeek responses (including streams). Ensured accurate cost calculation using registry metadata.

internal/providers/helpers.go

@@ -122,6 +122,33 @@ func BuildOpenAIBody(request *models.UnifiedRequest, messagesJSON []interface{},
 	return body
 }
 
+type openAIUsage struct {
+	PromptTokens            uint32 `json:"prompt_tokens"`
+	CompletionTokens        uint32 `json:"completion_tokens"`
+	TotalTokens             uint32 `json:"total_tokens"`
+	PromptTokensDetails     *struct {
+		CachedTokens uint32 `json:"cached_tokens"`
+	} `json:"prompt_tokens_details"`
+	CompletionTokensDetails *struct {
+		ReasoningTokens uint32 `json:"reasoning_tokens"`
+	} `json:"completion_tokens_details"`
+}
+
+func (u *openAIUsage) ToUnified() *models.Usage {
+	usage := &models.Usage{
+		PromptTokens:     u.PromptTokens,
+		CompletionTokens: u.CompletionTokens,
+		TotalTokens:      u.TotalTokens,
+	}
+	if u.PromptTokensDetails != nil && u.PromptTokensDetails.CachedTokens > 0 {
+		usage.CacheReadTokens = &u.PromptTokensDetails.CachedTokens
+	}
+	if u.CompletionTokensDetails != nil && u.CompletionTokensDetails.ReasoningTokens > 0 {
+		usage.ReasoningTokens = &u.CompletionTokensDetails.ReasoningTokens
+	}
+	return usage
+}
+
 func ParseOpenAIResponse(respJSON map[string]interface{}, model string) (*models.ChatCompletionResponse, error) {
 	data, err := json.Marshal(respJSON)
 	if err != nil {
@@ -132,6 +159,16 @@ func ParseOpenAIResponse(respJSON map[string]interface{}, model string) (*models
 	if err := json.Unmarshal(data, &resp); err != nil {
 		return nil, err
 	}
+
+	// Manually fix usage because ChatCompletionResponse uses the unified Usage struct
+	// but the provider might have returned more details.
+	if usageData, ok := respJSON["usage"]; ok {
+		var oUsage openAIUsage
+		usageBytes, _ := json.Marshal(usageData)
+		if err := json.Unmarshal(usageBytes, &oUsage); err == nil {
+			resp.Usage = oUsage.ToUnified()
+		}
+	}
 	
 	return &resp, nil
 }
@@ -156,6 +193,14 @@ func ParseOpenAIStreamChunk(line string) (*models.ChatCompletionStreamResponse,
 		return nil, false, fmt.Errorf("failed to unmarshal stream chunk: %w", err)
 	}
 
+	// Handle specialized usage in stream chunks
+	var rawChunk struct {
+		Usage *openAIUsage `json:"usage"`
+	}
+	if err := json.Unmarshal([]byte(data), &rawChunk); err == nil && rawChunk.Usage != nil {
+		chunk.Usage = rawChunk.Usage.ToUnified()
+	}
+
 	return &chunk, false, nil
 }
 
@@ -210,24 +255,27 @@ func StreamGemini(ctx io.ReadCloser, ch chan<- *models.ChatCompletionStreamRespo
 				return err
 			}
 			
-			if len(geminiChunk.Candidates) > 0 {
+			if len(geminiChunk.Candidates) > 0 || geminiChunk.UsageMetadata.TotalTokenCount > 0 {
 				content := ""
 				var reasoning *string
-				for _, p := range geminiChunk.Candidates[0].Content.Parts {
-					if p.Text != "" {
-						content += p.Text
-					}
-					if p.Thought != "" {
-						if reasoning == nil {
-							reasoning = new(string)
+				if len(geminiChunk.Candidates) > 0 {
+					for _, p := range geminiChunk.Candidates[0].Content.Parts {
+						if p.Text != "" {
+							content += p.Text
+						}
+						if p.Thought != "" {
+							if reasoning == nil {
+								reasoning = new(string)
+							}
+							*reasoning += p.Thought
 						}
-						*reasoning += p.Thought
 					}
 				}
 				
-				finishReason := strings.ToLower(geminiChunk.Candidates[0].FinishReason)
-				if finishReason == "stop" {
-					finishReason = "stop"
+				var finishReason *string
+				if len(geminiChunk.Candidates) > 0 {
+					fr := strings.ToLower(geminiChunk.Candidates[0].FinishReason)
+					finishReason = &fr
 				}
 
 				ch <- &models.ChatCompletionStreamResponse{
@@ -242,7 +290,7 @@ func StreamGemini(ctx io.ReadCloser, ch chan<- *models.ChatCompletionStreamRespo
 								Content:          &content,
 								ReasoningContent: reasoning,
 							},
-							FinishReason: &finishReason,
+							FinishReason: finishReason,
 						},
 					},
 					Usage: &models.Usage{